Alumina-based ceramic compositions have been used for cutting tools, nozzles and in similar applications, where high performance in wear resistance, hardness and fracture toughness are requirements.
An earlier ceramic cutting tool composition is described, for instance, in U.S. Pat. No. 4,366,254 issued to Rich et al. on Dec. 28, 1982, essentially consisting of alumina and zirconia in which refractory metal carbides, nitrides and carbonitrides are homogeneously dispersed. The composition further contains less than 1 percent iron group metals. Landingham in U.S. Pat. No. 4,745,091 issued on May 17, 1988, describes another wear resistant composition containing a predominantly alumina matrix supplemented with other oxides such as zirconia, magnesia and hafnia, the oxides being further modified with silicon nitride or aluminum nitride, and having dispersed within the above two phases small particles of carbides, nitrides and/or carbonitrides of refractory metals. Yet another composition for cutting inserts is described by Iyori et al. in U.S. Pat. No. 4,543,343 issued on Sep. 24, 1985, which comprises titanium carbide and/or titanium boride particles dispersed uniformly in alumina and zirconia containing matrix. The composition of Iyori et al. also contains stabilizing and sintering aids, such as magnesium oxide, yttria and aluminium nitride.
It is to be noted that the above described compositions provided hard and substantially wear resistant ceramic articles, however, the cutting inserts made of such compositions were still subject to chipping and crack formation due to thermal stress. Further cutting insert developments were intended to overcome the above shortcomings by incorporating carbide and nitride whiskers into the oxide matrix.
U.S. Pat. No. 4,852,999 issued to Mehrotra et al. on Aug. 1, 1989, describes a ceramic composition having an alumina matrix, in which single crystal titanium carbide whiskers have been dispersed. The size of the whiskers ranges between 0.25 and 3 .mu.m cross-sectional width, and according to Mehrotra et al. may have lengths up to 150 .mu.m. The composition of Mehrotra et al. contains zirconia as the preferred sintering aid. Brandt et al in U.S. Pat. No. 4,867,761 issued on Sep. 19, 1989, teach a similar ceramic cutting insert consisting of alumina-zirconia matrix in which whiskers of carbides, nitrides and/or borides of titanium and/or zirconium are homogeneously dispersed. The composition of Brandt et al. may also contain silicon carbide whiskers. The whiskers of Brandt et al. are monocrystals, having 0.5-10 .mu.m diameter and 2.5-100 .mu.m length, and aspect ratio ranging between 5-10. Yet other similar compositions containing additionally chromium compounds, or whiskers of tantalum compounds are described in U.S. Pat. Nos. 4,804,645 and 5,141,901 issued to Ekstrom on Feb. 14, 1989 and Brandt on Aug. 25, 1992, respectively. The whiskers in the above cutting tool compositions are added to increase strength and chipping resistance. Moreover, it is known that these substances added in the form of whiskers, increase heat conductivity and thereby reduce thermal cracking of the alumina matrix.
It is, however, known that whiskers, especially refractory substance containing whiskers, may be highly carcinogenic and environmentally harmful. Whiskers which are considered harmful have usually less than 4 .mu.m diameter. The harmful effects of the whiskers may be manifested during the preparation of the composition, as well as during usage of the sintered ceramic article made of a whisker containing composition.
The object of the present invention is to provide an alumina based composition for hard, sintered, wear resistant ceramic articles, which are also resistant to chipping and thermal cracking, while substantially eliminating the harmful side effects of ceramic whiskers.